High gain antennas are widely useful for communication purposes such as radar, television receive-only (TVRO) earth station terminals, and other conventional sensing/transmitting uses. In general, high antenna gain is associated with high directivity, which in turn arises from a large radiating aperture.
High gain antenna systems are often used in connection with TVRO systems such as those in the circularly polarized direct broadcast (DBS) band and in other linearly polarized systems. TVRO systems have been available since the early 1980s to those desiring to watch television via satellite delivered signals in their homes. A common method for achieving a large radiating aperture in TVRO applications is by the use of parabolic reflectors fed by a feed arrangement located at the focal point or focus of the parabolic reflector. Typically, large mesh or solid parabola-type antennas (i.e. backyard dishes) are placed in the yard of the consumer. Such parabolic dishes are often motorized so as to enable rotational movement along particular spatial arcs in which satellites are disposed thereby allowing the homeowner or consumer to view any one of a number of different satellites, one at a time. Unfortunately, movement of such parabolic antennas via the motor from one satellite to another is time consuming (i.e. it may take up to two minutes or more in some instances for the motor to move a typical parabola dish-type antenna from one extreme of the arc of satellites to the other) and subject to mechanical breakdown.
Motorized parabolic antenna systems also tend to be bulky, noisy, and subject to high maintenance requirements due to their abundance of moving parts. As stated above, most such parabolic antennas can only receive one satellite signal at a time. This is because typically a parabolic antenna reflects and concentrates the received signal to its focal point. A feed is mounted at the focal point to receive the signal and direct it to an amplifier/down converter which then directs the signal to the receiver in the home. Thus, depending upon what direction the dish is oriented, one satellite signal is focused into the focal point feed at a time.
Some prior art parabolic antennas have included multiple feeds near the center of the dish so as to enable the homeowner to receive multiple satellite signals simultaneously. Unfortunately, the angular range of such multi-feed systems is limited and such multi-feed antennas typically experience a signal loss because the multi-feeds are not directly in the center (i.e. focal center) of the dish but are only in its general proximity. Additionally, parabolic antennas often suffer structure required to support the feed, this often adversely affecting the illumination of the aperature and thereby perturbing the far-field radiation pattern.
Modern antenna systems have found increasing use of antenna arrays for high gain purposes. Phased array antennas often consist of a single output port and a plurality of stationary antenna elements which are fed coherently and use variable phase or time-delay control at each element to scan a beam to given angles in space. Such systems are highly expensive and are generally for this reason not used in TVRO applications. Variable amplitude control is sometimes also provided for pattern shaping. Single beam phased arrays are sometimes used in place of fixed aperture parabolic antennas, because the multiplicity of antenna elements allows more precise control of the radiation pattern thus resulting in lower side lobes and precise pattern shaping. The primary reason for the widespread use of such phased array antennas is to produce a directive beam that can be repositioned (scanned) electronically as opposed to the mechanical repositioning requirements of motorized parabolic antennas.
While phased arrays often have a single output port, multiple beam antenna systems have a multiplicity of output ports, each corresponding to a beam with its peak at a different angle in space. Typical systems utilizing such multiple beam technology and needing simultaneous, independent beams include multiple-access satellite systems and a variety of ground-based height-finding radars. Generally, multiple beam array antenna systems utilize a switching network that selects a single beam or a group of beams as required for specific applications via a generic lens or reflector. Other applications for multiple beam arrays include their use in the synthesis of shaped patterns where the beams are the constituent beams that combine to make up the shaped pattern, as in the commonly known Woodward-Lawson procedure. In still other cases, multiple beam arrays are used as one component of scanning systems such as the use of a multiple beam array feed for a reflector or lens system.
With the advent of higher power Ku band and direct broadcast satellites (DBS), it has become possible to manufacture array antennas having a diameter of less than about one meter. DBS is a term generally used for direct satellite to home transmissions. Such small high gain antennas have clear aesthetic advantages over bulky parabolic antennas.
Array antennas generally include an array (or plurality of elements or of subarrays of elements) of ordinarily identical antenna elements, each of which has a lower gain than the gain of the array. The antennas (or elements) are arrayed together and fed with an amplitude and phase distribution which establishes the far-field radiation pattern. Since the phase and power applied to each element of the array can be individually controlled, the direction of the beam (transmitting and receiving) can be controlled by controlling the amplitude and phase applied to each element in phased array systems. In multiple beam systems, reflectors or lenses are used to control the beam. A salient advantage of array antennas is clearly the ability to scan the beam or beams electronically without moving the mass of a reflector as is required in prior art parabolic-type antennas. A widespread problem of conventional phased array antennas and parabolic-type antennas is that they are limited to viewing one satellite at a time without experiencing reduced power or gain.
Existing satellites currently in orbit generally transmit two different and distinctive types of signals, namely circularly polarized (right and left-handed), and linearly polarized (horizontal and vertical). Accordingly, typical helical antenna elements making up array antennas may be wound in either the right-handed or left-handed directions. Helical antenna elements having right-handed windings or turns thereon may receive right-handed circularly polarized signals from right-handed satellites, but are eternally blind to left-handed circularly polarized satellite signals. This is also the case with left-handed helical antenna elements, such elements having the ability to received left-handed circularly polarized signals from satellites but being blind to satellite emitting right-handed circularly polarized signals.
Thus, conventional array antennas having only a plurality of left-handed circularly polarized antenna elements are blind to right-handed transmitting satellites, and arrays having only right-handed wound elements are blind to satellites transmitting left-handed circularly polarized signals. Therefore, consumers, in view of the limitations of the prior art, must decide whether they wish to view right-handed or left-handed circularly polarized signals in determining which type of antenna array to purchase (i.e. right-handed or left-handed) because conventional arrays are generally either right or left-handed.
While conventional multiple beam array antenna systems can receive beams from different satellites, such antennas cannot simultaneously receive signals from different satellites at substantially the same frequency where the satellites have different polarizations such as those of right and left handed circular polarization.
Accordingly, the need arises for an array antenna system having the ability to receive both right-handed and left-handed circularly polarized satellite signals, as well as linearly polarized signals (horizontal and vertical). Additionally, it would satisfy a long felt need in the art if such an antenna system were to be able to simultaneously receive signals from multiple satellites without substantial reduction in antenna directivity or gain, the received signals being any combination of right-handed, left-handed, or linear polarizations.
Currently, communication satellites re-broadcasting television signals to television receive-only (TVRO) earth stations from geostationary orbits over the equator are spaced apart by predetermined degrees of longitude (e.g. 4.degree.). Such angular spacing between satellites places severe requirements on TVRO antennas. In order to satisfactorily discriminate against interference from adjacent satellites that are re-using the same frequency band and polarization, antennas having high directivity and narrow beamwidths are required. Satisfying such requirements with conventional parabolic antennas necessitates the use of reflectors having very large diameters, this, of course, being undesirable. Clearly, there is also a need for a small, cost effective, array antenna system that is highly responsive to signals arriving from a primary receiving direction (e.g. satellite) but which can effectively nullify signals and noise arriving from other directions which differ from the primary receiving direction by a very small angle.
U.S. Pat. No. 4,845,507 discloses a modular radio frequency array antenna system including an array antenna and a pair of steering electromagnetic lenses. The antenna system of this patent utilizes a large array of antenna elements (of a single polarity) implemented as a plurality of subarrays driven with a plurality of lenses so as to maintain the overall size of the system small while increasing the overall gain of the system. Unfortunately, the array antenna system of this patent cannot simultaneously receive both right-hand and left-handed circularly polarized signals, and furthermore cannot simultaneously receive signals from different satellites wherein the signals are right-handed circularly polarized, left-handed circularly polarized, linearly polarized, or any combination thereof.
U.S. Pat. No. 5,061,943 discloses a planar array antenna assembly for reception of linear signals. Unfortunately, the array of this patent, while being able to receive signals in the fixed satellite service (FSS) and the broadcast satellite service (BSS) at 10.75 to 11.7 GHz and 12.5 to 12.75 GHz, respectively, cannot receive signals (without significant power loss and loss of polarization isolation) in the direct broadcast (DBS) band, as the DBS band is circular (as opposed to linear) in polarization.
U.S. Pat. No. 4,680,591 discloses an array antenna including an array of helices adapted to receive signals of a single circular polarization (i.e. either right-handed or left-handed). Unfortunately, because satellites transmit in both right and left-handed circular polarizations to facilitate isolation between channels and provide efficient bandwidth utilization, the array antenna system of this patent is blind to one of the right-handed or left-handed polarizations because all elements of the array are wound in a uniform manner (i.e. the same direction).
It is apparent from the above that there exists a need in the art for a multiple beam array antenna system (e.g. of the TVRO type,) which is small in size, cost effective, and modular so as to increase gain without significantly increasing cost. There also exists a need for such a multiple beam array antenna system having the ability to receive each of right-handed circularly polarized signals, left-handed circularly polarized signals, and linearly polarized signals. Additionally, the need exists for such an antenna system having the potential to simultaneously receive signals from different satellites, the different signals received being of the right-handed circularly polarized type, left-handed circularly polarized type, linearly polarized typed, or combinations thereof. It is the purpose of this invention to fulfill the above-described needs in the art, as well as other needs apparent to the skilled artisan from the following detailed description of this invention.
Those skilled in the art will appreciate the fact that array antennas are reciprocal transducers which exhibit similar properties in both transmission and reception modes. For example, the antenna patterns for both transmission and reception are identical and exhibit approximately the same gain. For convenience of explanation, descriptions are often made in terms of either transmission or reception of signals, with the other operation being understood. Thus, it is to be understood that the array antennas of the different embodiments of this invention to be described below may pertain to either a transmission or reception mode of operation. Those of skill in the art will also appreciate the fact that the frequencies received/transmitted may be varied up or down in accordance with the intended application of the system.
Those of skill in the art will also realize that right and left-handed circular polarization may be achieved via properly summing horizontal and vertical linearly polarized elements.